The present invention relates to a semiconductor laser device and a method for producing the same, in particular to a semiconductor laser device used for a light source of optical discs and so on, and a method for producing the same.
There has hitherto been an end-face emitting type semiconductor laser device for optical discs. Such a semiconductor laser device is required to generate a high output to write information to an optical disc at a high speed. However, there is a problem that degradation occurs at laser beam-emitting end faces when high-output operation is performed. In order to suppress the degradation at the laser beam-emitting end faces, a structure called “window structure” is generally used. The window structure is formed in regions in proximity of laser beam-emitting end faces of an active layer by intermixing the regions of the active layer (hereinafter these regions are referred to as “window regions”). The window structure is formed in order to broaden the energy band gap of quantum well layers in the window regions and thereby reduce absorption of light in the window regions. Since the window structure is constructed such that absorption of light hardly takes place, it is possible to prevent degradation of the laser beam-emitting end faces due to strong laser beams, and also possible to prevent a reduction in the emission power of laser beams.
Incidentally, in the window structure, if a current flows through the window regions of the active layer, light different from that in an inner region of the active layer is generated, which becomes a factor for degradation of the end faces. Accordingly, in order to prevent a current from flowing through the window regions, it is required that a current non-injection structure be added to the semiconductor laser device.
In order to show an example of a conventional end-face current non-injection structure, the structure of a first semiconductor laser device disclosed in JP-A-03-153090 is shown in FIGS. 10A and 10B. FIG. 10A is a perspective view of the first semiconductor laser device, and FIG. 10B is a cross-sectional view taken along line 10B—10B of FIG. 10A.
With regard to current injection region A of FIG. 10A in the first semiconductor laser device, an n-type GaInP buffer layer 2, an n-type AlGaInP cladding layer 3, a GaInP active layer 4, a p-type AlGaInP cladding layer 5, a p-type GaInP intermediate band gap layer 6, an n-type GaAs block layer 7, and a p-type GaAs contact layer 8 are stacked in this order on an n-type GaAs substrate 1, as shown in FIG. 10B.
On the other hand, in current non-injection regions B of FIG. 10A in the first semiconductor laser device, as seen from a laser beam-emitting end face 50 of FIG. 10A, the p-type GaAs contact layer 8 is directly formed on the p-type AlGaInP cladding layer 5, and the p-type GaInP intermediate band gap layer 6 is eliminated.
With regard to the manner in which a current flows through a semiconductor laser device (voltage-current characteristic), comparison was made between a semiconductor laser device which is made of only the current injection region A and a semiconductor laser device which is made of only the current non-injection region B. The results thereof are shown in FIG. 11. In FIG. 11, solid line A indicates the voltage-current characteristic of the semiconductor laser device made of only the current injection region A and broken line B indicates the voltage-current characteristic of the semiconductor laser device made of only the current injection region B. When a voltage of 2.5 V is applied, a current flows through the semiconductor laser device made of only the current injection region A, as shown by solid line A in FIG. 11, while a current does not flow through the semiconductor laser device made of only the current non-injection region B.
Using FIGS. 12A and 12B, a phenomenon that a current hardly flows at a junction interface between semiconductor layers will be described. In FIGS. 12A and 12B, the horizontal axis shows a distance from the p-type AlGaInP cladding layer 5 to the p-type GaAs contact layer 8 (in a direction perpendicular to the n-type GaAs substrate 1), while the vertical axis shows an energy level of the semiconductor laser device. FIG. 12A refers to the current injection region A and FIG. 12B refers to the current non-injection region B. In FIG. 12, Ec shows an energy level of the conduction band (electrons), Ev shows an energy level of the valence band (holes), and a difference between Ec and Ev shows an energy band gap.
In the first semiconductor laser device, the p-type GaInP intermediate band gap layer 6, which has an energy level intermediate between the levels of the p-type AlGaInP cladding layer 5 and the p-type GaAs contact layer 8, is provided in the current injection region A. Therefore, as shown in FIG. 12A, energy barriers ΔEa1 and ΔEa2, which are generated due to a difference between energy band gaps can be reduced and thus flow of current (holes) can be made smooth.
On the other hand, in the first semiconductor laser device, because the p-type AlGaInP cladding layer 5 is in direct contact with the p-type GaAs contact layer 8, an energy barrier ΔEb generated due to a difference between energy band gaps can be made large. Thus, flow of current (holes) can be prevented. In this manner, the first semiconductor laser device prevents a current from flowing through the window regions.
However, when producing the first semiconductor laser device, a process of selectively removing only the p-type GaInP intermediate band gap layer 6 in proximity of laser beam-emitting end faces is required in order to form current non-injection regions. This process has a problem, which will be described below using FIGS. 13A and 13B. FIGS. 13A and 13B are schematic cross-sectional views showing the conventional current non-injection region.
In the first semiconductor laser device, a p-type GaInP intermediate band gap layer 41 shown in FIG. 13A is usually removed by wet etching. In the case where a liquid containing bromine, which is a typical etchant, is used, a p-type AlGaInP cladding layer 42 shown in FIG. 13A is also etched. Thus, as shown in FIG. 13B, the thickness of the p-type AlGaInP cladding layer 42 is reduced in the current non-injection region. Because laser beams spread to an upper end portion of the p-type AlGaInP cladding layer 42, the reduction in the thickness of the p-type AlGaInP cladding layer 42 deteriorates the function of confining laser beams in an active layer, which causes absorption of light, resulting in deterioration of emission power.
Further, in the case where a so-called real guide structure, which reduces absorption of light, is constructed by replacing the n-type GaAs block layer 7 of the first semiconductor laser device shown in FIG. 10A and FIG. 10B with an n-type AlInP block layer, there is also a problem that in a process step of etching the p-type GaInP intermediate band gap layer 41, both the n-type AlInP block layer 133 and the p-type AlGaInP cladding layer 132 forming a ridge are also etched. Describing this in more detail, when the real guide structure is adopted, because the n-type AlInP block layer 43 is easy to etch in the vicinity of ridge side surfaces 42a (see FIG. 13A) of the p-type AlGaInP cladding layer 42 where the n-type AlInP block layer 43 has crystal quality different from that on a flat surface, the ridge of the p-type AlInP cladding layer 132 and a boundary surface of the n-type AlInP block layer 133 are curved and deformed, as shown in FIG. 13B. Consequently, light is easily absorbed in the vicinity of the laser beam-emitting end faces of the semiconductor laser device. In FIG. 13B, reference numeral 45 indicates a portion of the n-type AlInP block layer to be etched in the process of etching the p-type GaInP intermediate band gap layer 41, while reference numeral 46 indicates a portion of the p-type AlGaInP cladding layer to be etched in the process step of etching the p-type GaInP intermediate band gap layer 41.
A second semiconductor laser device disclosed in JP-A-9-293928, which is shown in FIG. 14, has the following problem.
In the second semiconductor laser device, an n-type AlGaInP cladding layer 22, an active layer 23, a p-type AlGaInP cladding layer 24, a p-type GaInP layer are stacked in this order on a substrate 21. Then, a series of process steps for intermixing portions in proximity of laser beam-emitting end faces of the active layer 23 (details of which are herein omitted) is conducted. Furthermore, window structures 30 having an increased band gap are formed in the vicinity of the laser beam-emitting end faces of the active layer 23. In the second semiconductor laser device, after the window structures 30 are formed, a ridge, a current blocking layer 26, and a contact layer 32 are formed. Then, for the purpose of preventing a reactive current from flowing through the window regions, resistance-increased proton-injected regions 33 are formed in the contact layer 32 on the sides of the laser beam-emitting end faces by proton injection method.
In the second semiconductor laser device, the proton injection method is used, but injection of protons causes defects in crystals. Thus, there is a problem that crystal defects increase during the operation of the semiconductor laser device, resulting in deterioration of the semiconductor laser device. On the other hand, if protons having a weak energy are injected in order to suppress the deterioration of the semiconductor laser device, the sufficient current non-injection effect cannot be achieved.